Optoelectronic distance measuring devices, such as phase-principle laser range finders, have been widely used in construction, interior decoration and other fields with the aid of their high accuracy of measurement. The general principles of measurement are as follows: an emitter emits a modulated measuring beam onto an object to be measured, the beam is then reflected or scattered by the object to be measured, and the reflected beam is picked up by a optoelectronic receiver. Thus, thus the distance from the object to be measured is obtained on the basis of the phase position shift of the modulated measuring beam relative to the emitter. Usually, an avalanche photodiode is used as the optoelectronic receiver for receiving the modulated measuring beam reflected by the object.
FIG. 1 shows a circuit diagram of the optoelectronic distance measuring device which measures the distance based on the phase principle. A mixer signal and an initial frequency signal with the same frequency and the same phase are generated by a phase locked loop (PLL) circuit 11′. The initial frequency signal and a low frequency signal generated by a microprocessor 12′ are transmitted onto a quadrature modulator 13′ and a frequency modulation signal is then produced in the quadrature modulator 13′ and output therefrom. The frequency modulation signal is then amplified by a power amplifier 14′, and is superposed on the laser emitter 15′ for frequency-modulation of the measuring beam. The emitter 15′ emits a frequency-modulated measuring beam onto an object 16′ to be measured. The avalanche photodiode 17′, which is used as a direct mixer simultaneously, receives the frequency-modulated measuring beam reflected by the object 16′ to be measured. The mixer signal is mixed with signals generated according to the received frequency-modulated measuring beam reflected by the object to be measured in the avalanche photodiode 16′ and leads to output signals, the output signals are amplified by a transresistance amplifier 18′ and filtered out by a low-pass filter 19′, then a low-frequency measuring signal is produced. The low-frequency measuring signal contains a phase information which is used for calculating a distance to the object to be measured.
Although the avalanche photodiode has the advantage of high amplification and sensitivity, it is necessary to apply a high, temperature-dependent operating voltage to the avalanche photodiode. Usually a variable, temperature-dependent bias voltage is applied to it. As a result, a capacity of the avalanche photodiode varies with the bias voltage which changes in accordance with temperature, and an unexpired phase drift is produced. The phase drift is thus added to the above-mentioned low-frequency measuring signal which contains the phase information used for calculating the distance to be measured, and hence a measuring error is produced.
In the prior art, an internal reference optical path with a predetermined length is provided in the distance measuring device so as to eliminate the phase drift produced by the avalanche photodiode. A mechanical switching device used for switching between an external measuring optical path and the internal reference optical path is arranged in the emitting optical path of the measuring beam. The avalanche photodiode receives the modulated measuring signal passing through the external measuring optical path and the modulated reference signal passing through the internal reference optical path successively and produces a low-frequency measuring signal and a low-frequency calibration signal, respectively. The low-frequency measuring signal and the low-frequency calibration signal both contain the phase drift produced by the avalanche photodiode and the phase drift is then counteracted by subtracting the phase of the calibration signal from the phase of the measuring signal so that the measuring error is eliminated. The measuring signal and the calibration signal reach the avalanche photodiode successively and alternately by the mechanical switching, which can take many times during a measurement process. Repeated mechanical switching during the measurement process does however result in high mechanical load and considerable wear of the moving parts. Furthermore, the mechanical switching device and the internal optical path make the structure of the distance measuring device complicated which leads to high manufacturing costs and a large weight and volume. All of these are disadvantageous for the miniaturization of the distance measuring device.
Two optoelectronic receivers are alternatively used in some range finders for receiving the reflected measuring signal and the reference signal simultaneously. However, the additional expensive optoelectronic receiver also increases the manufacturing costs of the range finder.